Integrated circuits are created from a semiconductor wafer using various etching, doping, and depositing steps that are well known in the art of fabricating integrated circuit devices. The wafer may be comprised of a number of integrated circuit dies that each represent a single integrated circuit chip. Modern high-performance integrated circuits are comprised of millions of transistors that perform functions such as random access memory, central processing communications, etc. Each of these transistors must be interconnected with electrically-conducting elements. A typical modern integrated circuit now contains multiple layers of these conductor elements. Due to the size constraints of placing millions of conducting elements on a chip that has an area of only a few square centimeters, the connecting elements themselves have become very small, and the distance that separates conducting elements has shrunk as well. A state-of-the-art integrated circuit produced today has a conductor width of 0.25 microns and conductor spacing of 0.25 microns.
Because of the small spacing, many electrical performance problems are now arising in integrated circuits. The reduced spacing results in increased electrical capacitance, which causes capacitative interconnect delay and can slow down the operation speed of the circuit. Capacitance also causes cross-talk which can result in signal errors being generated if the problem is not solved. In addition, it is also desirable to reduce the capacitance in order to reduce the amount of power that the integrated circuit requires to operate.
Since the dimensions of the integrated circuit are constrained, for example, at 0.25 microns for the current device technology generation and decreasing to as little as 0.07 microns in 10 years, the only way to reduce the capacitance between the conducting elements is to use an insulative material with a low dielectric constant. Conventional semiconductor fabrication commonly uses silicon dioxide as a dielectric, which has a dielectric constant of about 3.9. The lowest possible or ideal dielectric constant is 1.0, which is the dielectric constant of a vacuum, whereas air has a dielectric constant of less than 1.001.
While air and vacuum have acceptably low dielectric constants, there is another important factor known as dielectric strength which must be taken into consideration. Dielectric strength is typically referred to in one of two ways. One way is breakdown voltage or breakdown field strength. Breakdown field strength is a property with units of volts per unit length at which an insulative material does not insulate, breaks down and results in a short circuit. To calculate the required minimum breakdown field strength for a modem integrated circuit, one takes the operating voltage of the circuit and divides it by the separation distance between adjacent conducting elements. For example, in a 0.25 micron technology integrated circuit that operates at a voltage of 3.3 volts, the minimum breakdown field strength required is 3.3 divided by 0.25 microns, which equals 13.2 volts per micron, or 0.132 MV/cm. Typical safety margins are several times this or a minimum of about 0.5 MV/cm.sup.2. The breakdown field strength of air is less than 1 volt per micron.
The second component of dielectric strength is leakage current. Leakage current is low level current flux through an insulator of field strength less than the breakdown field strength. A typical requirement for an integrated circuit is a leakage current density less than 2.times.10.sup.-8 amps per square centimeter and an applied electric field strength of 0.05 MV/CM.
It is therefore advantageous to provide a material with both a low dielectric constant and a high dielectric strength. Polytetrafluoroethylene (PTFE) is such a material. It has a dielectric constant of 2.0, which is the lowest dielectric constant known for any non-porous material that has a thermal stability high enough to withstand the rigors of integrated circuit manufacturing processes. Bulk PTFE is also known to have very high breakdown field strength and very low leakage current density. However, previous attempts to deposit thin sub-micron films of PTFE onto a silicon wafer have resulted in materials that lack desired dielectric strength. In forming these films, the film is usually sintered to fuse the PTFE particles together to form a coherent film, which is then cooled. Applicants have determined that slowly cooling a sintered thin PTFE film results in a porous material having less than acceptable dielectric strength. It would be advantageous to provide an improved process for depositing and processing the PTFE film which results in high dielectric breakdown field strength and low leakage current density.